In PET imaging, a positron emitting isotope is disposed within an object being imaged. Typically, the positron emitting isotope is included in a chemical tracer, which is thereby labeled with the isotope. Positrons emitted by the isotope rapidly encounter an electron within the object and annihilate, thereby creating a pair of oppositely directed 511 keV gamma ray photons. Coincidence detection of the 511 keV photons is employed in an imaging detector array to provide spatially resolved imaging of the annihilation events (i.e., the PET image).
Positron emitting isotopes can be roughly divided into so-called “clean” emitters and “dirty” emitters depending on whether or not positron emission is accompanied by other kinds of radioactive emission. For clean emitters, there is relatively little non-positron emission (e.g., 10% or less), while dirty positron emitters can have greater non-positron emission (e.g., more than 10%). Since the non-positron radiation emitted by dirty positron emitters can cause undesirable background noise in a PET system, clean positron emitters are preferred in conventional PET systems. Some attention has been paid to correcting PET imaging results for the presence of background noise from dirty positron emitters, e.g., as considered in U.S. Pat. No. 7,777,189 and in US 2008/0283758.
Some attention has also been paid to exploiting dirty emitters to provide more information from a PET system than can be obtained using conventional clean positron emitters. An example of this approach is described in an article by Andreyev et al. (“Feasibility study of dual-isotope PET”, IEEE Nuclear Science Symposium 1020 Conference Record pp 2108-2111). In this work, two isotopes are used simultaneously in PET, where the first isotope is a clean positron emitter, and the second isotope is a dirty positron emitter that provides a prompt gamma ray in addition to a positron. Signals from the two isotopes are distinguished by determining whether a detected event is a 2-photon event (attributed to the clean emitter) or a 3-photon event (attributed to the dirty emitter).